The present invention relates generally to field emitters and more particularly to field emitters for a flat panel display.
Numerous researchers have been trying to make reliable field emitters for a flat panel display. It is a very difficult task.
Typically, a field emitter has an emitter that has a sharp tip, with a gate adjacent to the tip. The field emitter is inside a partial vacuum with a fluorescent screen above the gate. The screen is usually at a high positive voltage. When a selected potential difference is applied between the gate and the emitter, electrons are extracted from the tip, and are attracted to the screen by its high positive voltage to generate light on the screen.
One prior art approach generates emitters in the shape of a cone. The tip of each cone is in the middle of its corresponding aperture, with the edge of the aperture in close proximity to the cone. Each aperture serves as the gate of its corresponding cone. Typically, a pixel on a flat panel display is generated by an array of such emitters. A general discussion on such emitters or cathodes can be found in "Physical properties of thin-film field emission cathodes with molybdenum cones," written by C. A. Spindt et al., and published in the Journal of Applied Physics, volume 47, number 12, December 1976.
A typical potential difference between a cone emitter and its corresponding gate is about 100 volts, and the aperture in the shape of a hole has a dimension of about 1 micron. Such a high potential difference not only is a challenge to the corresponding electrical drivers, it also consumes more power than desired, especially if the display is operated by battery. There have been attempts to reduce the potential difference between the gate and its corresponding emitter to less than thirty volts by reducing the dimension of the hole to less than 0.1 micron. Such attempts have not been very successful. Imagine a flat panel display with millions of such emitters across a 10 inches by 10 inches area. It is very difficult and expansive to perform sub-half-micron lithography across such a large surface area.
Also, in the above-described cone and concentric gate field emitter configuration, the electron emission is exponentially proportional to the inverse of the tip diameter, which is one of the more difficult parameters to control in fabricating the field emitter. Due to the extremely sharp nature of the tip, a small change in the tip diameter may produce an unacceptable variation in the emitted electrons, leading to unacceptable non-uniformity in light emission across the display. Such non-uniformity may lead to flickering in electron emission.
Operating voltage on an emitter is proportional to the diameter of the tip of the cone, which can be as small as 100 angstroms, and is typically fabricated using expensive collimated-vacuum-deposition techniques. It is difficult to produce millions of such cones with their tip dimensions substantially the same over the extended area described above. One solution to this non-uniformity is to produce a large number of emitting cones for each pixel. Although one still has to produce the extremely fine tip for each cone, flickering in electron emission is reduced by statistical averaging. Another commonly used scheme to resolve the problem of non-uniformity is to add a current limiting resistor to each cone. The resistor for a cone produces a self-bias that is proportional to the amount of electrons emitted from the cone per unit time--the emission current. The self-bias, in turn, reduces the electric field at the tip of the cone.
In fabricating the millions of field emitters in an array for a flat panel display, the gate and emitter conductors are usually arranged into rows and columns with insulators among them to make them individually addressable. Typically, the insulators among the conductors are left exposed, and are thus bombarded either by electrons emitted from the field emitters, or by electrons back-scattered from the phosphor screen and other positively charged surfaces. Electrons tend to accumulate on the insulator surfaces. This accumulation of the electrons on the exposed insulating surfaces over a prolonged period of time may build potential differences between the insulators and their adjacent conductors to a level higher than those that can be sustained by the insulators. Very often, catastrophic discharges of the accumulated electrons along the surfaces of the insulators will occur, leading to breakdowns of the insulators or destruction of the material locally. This, in turn, will render the device inoperable.
The difficulty in lithography also limits the number of emitters that can be cramped into a single pixel, and limits the effectiveness of the statistical averaging to reduce the flicker in the electron emission. One technique used to increase the density of emission sites, without resorting to even finer and more expensive lithographic techniques, is the use of a line or edge emitter. Line emitter can be envisioned as formed by arranging point emitters with the cones lined up to form a linear array with no space between the tips of the cones. Again, in close proximity to each line is its corresponding gate, which has the structure of a slot, with the line emitter centered in the slot; and the dimension of the slot may be in the sub-micron range. Similar to the cones, the line emitters have a number of difficulties-the sub-micron lithography, the radius of curvature of the edge of each line possibly in the range of 100 angstroms, and many exposed dielectric surfaces to collect reflected electrons. It is difficult to create sharp edges consistently with uniform sharpness along each line to ensure uniform electron emission. If a certain part of an edge is sharper than other parts, electron-emission will be concentrated in that sharp area. Then one will get a dim line of light with a very bright dot. Such an embodiment is not preferred because the possibility of electric breakdown is much higher at that sharp area, and because this reduces the statistical averaging benefit of the line emitter. Moreover, the line emitter is usually separated from its gate by a dielectric layer. The dielectric layer must be thin in order to bring the emitter close to the gate to extract electrons. Any non-uniformity or pinholes in the dielectric layer might lead to dielectric breakdown, which will destroy the device.
Although the line emitter produces a much denser population of emission points or sites, the gain in electron emission may be nullified by a reduction in the electric field at emission sites along the edge of the line. Positioned next to one another, the emission sites shield one another to reduce the electric fields at the emission sites. This reduction in the fields causes the emission from individual site to decrease. To increase the emission current, one could increase the potential difference applied between the gate and the emitter, which would increase the power consumption of the device. Another way to increase the emission current is to reduce the distance between the emitter and the gate, which would increase the field in the dielectric layer separating the gate and the emitter. However, an increase in electric field would increase the chance of dielectric breakdown. The emission current could also be increased by reducing the radius of curvature of the emitters. However, this would add to the difficulty in lithography and thin film processing, further increasing the cost to fabricate the device.
In one implementation, the line emitters are metallic. The conceptual emission sites along such a line emitter are not physically separated from one another. One way to enhance uniform emission is to add individual current limiting resistors to these sites. However, with a metallic emitter, adding such resistors to these conceptual sites is physically impossible. Without the current limiting resistors for the individual sites, the electron emission from the edge would come from sites with smaller radii of curvature. This significantly reduces one of the advantages of line emitters, namely, the high density of emission sites.
One way to alleviate the increased difficulty in processing and lithography is to turn the vertically standing emitter structures into a planar, horizontal, stacked structure. Typically, in such a planar structure, gates are fabricated over and under the emitters to form a gate-emitter-gate stack. Electrons emitted from the horizontal edge or line emitter tend to travel in a horizontal direction. A coplanar anode is used to collect the emitted electrons. These structures may not be suitable for display applications because it is difficult for the remotely located screen of a display to efficiently collect electrons. Some other researchers improved the structure with an additional shielding layer under the lower gate, and a deflector electrode in front of the gate-emitter-gate stack. With appropriate negative voltages on the shield and the deflector, electrons initially in the horizontal direction are deflected to travel in a vertical direction toward the remotely located screen. Such improved structures have a number of defects. First, the deflector and the shield have to be very close to the stacked gate-emitter-gate structure in order to be effective; this requires the use of advanced lithography tools and techniques. Moreover, the voltage on the shield has to be negative enough, and the position of the shield has to be close enough to the emitter to deflect electrons. Such a shield being so close to the emitter is also very close to the gate. This might lead to arcing and dielectric breakdown, which will destroy the device.
It should be apparent from the foregoing that there is still a need for a field emitter that is not too difficult to build, with a high efficiency, capable to form a uniform electron beam across its corresponding pixel, and with a significantly reduced possibility of arcing and dielectric breakdown, which are two of the major causes for the destruction of the thin-film field emitter arrays.